As shown in FIG. 3, in a shoe press process, a shoe press mechanism comprising a looped shoe press belt 2 interposed between a press roll 1 and a shoe 5 is used, the press roll 1 and the shoe 5 provide a pressing region therebetween through which a transfer felt 3 and a wet paper web 4 are caused to pass so as to dewater the wet paper web 4.
As shown in FIG. 2, the shoe press belt 2 comprises an outer circumferential polyurethane layer 21 and an inner circumferential polyurethane layer 22 which are disposed respectively on both surfaces of a reinforcing fiber base 6 which is sealed (embedded) in the polyurethane layers, wherein the outer circumferential polyurethane layer 21 of the press roll side further has a number of concave grooves 24 defined in its surface, water which is squeezed from the wet paper web 4, when it is pressed in the pressing region, is held in the concave grooves 24, and then the held water is brought out of the pressing region as the shoe press belt rotates. Convex parts 25 on the outer circumferential polyurethane layer 21, which are formed at the press roll side, are required to have their improved mechanical properties such as wear resistance, crack resistance and flexural fatigue resistance, etc. against vertical pressing forces applied by the press roll 1 and wear and flexural fatigue of the shoe press belt in the pressing region.
For the above reasons, polyurethane, which has an excellent crack resistance, is widely used as the resin material of the outer circumferential polyurethane layer 21 of the shoe press belt 2.
For example, patent documents 1 and 2 disclose the shoe press belts comprising a reinforcing fiber base and polyurethane which are integrated each other, the polyurethane comprising an outer circumferential layer and an inner circumferential layer, and the reinforcing fiber base being embedded in the polyurethane, wherein,
a polyurethane of the outer circumferential layer is a polyurethane, which has a “JIS (Japanese Industrial Standards) “A” hardness” level ranging from 89 to 94 and which is obtained by curing a mixed composition of                an urethane prepolymer (HIPRENE L: trade name, manufactured by Mitsui Chemicals, Inc.) which is obtained by reacting tolylene diisocyanate (TDI) with polytetramethylene glycol (PTMG), and has a terminal isocyanate group, and        a curing agent including dimethylthiotoluenediamine,        wherein the urethane prepolymer and the curing agent are mixed so that the equivalent ratio (H/NCO) of an active hydrogen group (H) of the curing agent and an isocyanate group (NCO) of the urethane prepolymer has a value in the range of 1<H/NCO<1.15, andwherein a polyurethane of the inner circumferential layer is a polyurethane which is obtained by curing a mixed composition of        a urethane prepolymer which is produced by reacting 4,4′-methylene-bis(phenyl isocyanate) (MDI) with polytetramethylene glycol (PTMG), and has a terminal isocyanate group, and        a mixed curing agent which comprises 65 parts of dimethylthiotoluenediamine and 35 parts of polytetramethylene glycol (PTMG),        wherein the urethane prepolymer and the curing agent are mixed so that the equivalent ratio (H/NCO) of an active hydrogen group (H) of the curing agent and an isocyanate group (NCO) of the urethane prepolymer has a value in the range of 0.85≦H/NCO<1 (see patent document 1 and patent document 2).        
Further, patent document 3 discloses a shoe press belt for papermaking in which a reinforcing fiber base and a polyurethane are integrated each other, the polyurethane comprising an outer circumferential layer and an inner circumferential layer and the reinforcing fiber base being embedded in the polyurethane, wherein
polyurethane of the outer circumferential layer and the inner circumferential layer are polyurethane, which have a “JIS “A” hardness” level ranging from 94 to 95, formed by curing a mixed composition of                a urethane prepolymer (HIPRENE L: trade name, manufactured by Mitsui Chemicals, Inc.) which is produced by reacting tolylenediisocyanate (TDI) with polytetramethylene glycol (PTMG) and has a terminal isocyanate group, and        a curing agent including dimethylthiotoluenediamine,        wherein the urethane prepolymer and the curing agent are mixed so that the equivalent ratio (H/NCO) of an active hydrogen group (H) of the curing agent and an isocyanate group (NCO) of the urethane prepolymer has a value of 0.97.(see patent document 3).        
Patent document 4 discloses shoe press belts in which a reinforcing fiber base and a polyurethane layer are integrated each other, the reinforcing fiber base being embedded in the polyurethane, wherein
the polyurethane is a polyurethane, which has a “JIS “A” hardness” level ranging from 93 to 96, formed by curing a mixed composition of                a urethane prepolymer comprising an unreactive polydimethylsiloxane in a liquid form and obtained by reacting tolylene-diisocyanate (TDI) with polytetramethylene glycol (PTMG) and having a terminal isocyanate group, and        a curing agent selected from dimethylthiotoluenediamine (ETHACURE300: trade name, manufactured by Albemarle Corporation, USA) and 4,4′-methylene-bis-(2-chloroaniline) (MOCA),        wherein the urethane prepolymer and the curing agent are mixed so that the equivalent ratio (H/NCO) is in the range of 0.9≦H/NCO≦1.10, and        
the shoe press belt, which has a “JIS “A” hardness” level ranging from 90 to 93, is formed by curing a mixed composition of                a mixture of a polyurethane having a “JIS “A” hardness” level ranging from 90 to 93 and including unreactive polydimethylsiloxane in a liquid form and a polyurethane having a “JIS “A” hardness” level of 98 and free of unreactive polydimethylsiloxane in a liquid form, and        a curing agent of dimethylthiotoluenediamine,        wherein the mixture and the curing agent are mixed so that an equivalent ratio is in the range of 0.9≦H/NCO≦1.10 (see patent document 4).        
[Patent Document 1] JP, B, 3,698,984
[Patent Document 2] JP, A, 2005-120571
[Patent Document 3] JP, A, 2005-307421
[Patent Document 4] JP, A, 2006-144139
The shoe press belts disclosed in the patent documents 1 to 4 were measured by an inspecting apparatus, in which the opposite ends of a test piece of a belt were gripped by clamp hands, the clamp hands were reciprocally movable horizontally in a ganged fashion, the test piece had an evaluation surface facing a rotating roll, and the press shoe moved toward the rotating roll to press the test piece for measuring crack resistance thereof, while the test piece was subjected to a tensile force of 3 kg/cm and a pressure of 36 kg/cm2 by the inspecting apparatus, the clamp hands were reciprocally moved at a speed of 40 cm/sec., and the number of times that the clamp hands were reciprocally moved was measured until the test piece cracked. As a result, it was found that no crack developed in the test piece after the clamp hands were reciprocally moved 1,000,000 times.
In recent years, as the operating speed has increased, the shoe press belts have had an increased width of about 10 m, and the pressure applied in the pressing region has become higher to meet demands for higher paper productivity growth, the shoe press belts have been used in more and more severe environments, and therefore the various properties of the shoe press belts need to be improved further.
The object of the present invention is to provide a shoe press belt for papermaking which has more excellent mechanical properties in wear resistance, crack resistance, flexural fatigue resistance, etc.